1. Field of the Invention
The present invention relates to an image forming apparatus such as a thermal copying machine equipped with a scanner to irradiate light from an illumination lamp onto an original and read out image data from the reflected light.
2. Description of the Related Art
In a thermal color copying machine which uses a thermal ink ribbon with plural types of colors to make a color copy, for example, light is irradiated onto an original from an illumination lamp (light source), image data of the original is optically read out from the reflected light, and this image data is converted into color data corresponding to the individual inks of the thermal ink ribbon. In accordance with the color data, ink of the associated ink portion of the thermal ink ribbon is melted by a thermal head (heat-sensitive head) and transferred onto a sheet of paper (image-forming medium). in this manner, different colors are sequentially transferred onto the paper to make a color copy.
In some of the above type thermal color copying machines, the life expiration of the lamp is checked on the basis of the intensity of light from the lamp. In other words, the lamp's life expiration is checked on the basis of reduction in intensity of light. If the required intensity of light cannot be provided from a lamp in use any more, this lamp needs replacement.
Even if there is a sufficient intensity of light to scan a monochromatic original (two-colored (black and white) original), however, the ratio of chroma signals of light (relative ratio of red (R), green (G) and blue (B)) may actually vary due to reduction in color temperature of light which is caused by deterioration of the lamp. In this case, the color copying machine of the aforementioned type which checks the life expiration of the lamp based only on the intensity of light, cannot discriminate color imbalance caused by deterioration of the lamp. When a user uses a lamp whose life expiration has already been reached, without being aware of a change in ratio of R, G and B, therefore, the color tone of a color original cannot be reproduced with a high fidelity.